Scientists have shed light on the mystery of the preservation of the weapons of the Terracotta Army

The chrome-plated bronze weapons of the Terracotta Army, once considered the oldest antirust technology, stems from a decorative polish rather than a conservation technique, reveals a new study jointly conducted by researchers from the UCL and the Terracotta Army Museum.

The study, published today in Scientific reports, reveals that the chemical composition and characteristics of the surrounding soil, rather than chromium, can be responsible for the famous power of conservation of weapons.

The main author, Professor Marcos Martinón-Torres (University of Cambridge and formerly of the Institute of Archeology of UCL), commented: "The terracotta warriors and most Organic materials from the mausoleum were covered with a protective layer of lacquer before being painted with pigments – but, interestingly, bronze weapons. "

"We found a substantial chromium content in the lacquer, but only a trace of chromium in the pigments and the soil nearby, perhaps a contamination.The highest traces of chromium found on the bronzes are still on the parts of the weapon directly associated with now decomposed organic elements, such as spear sleeves and wooden and bamboo sword claws, which would also have been lacquer coated.It is clear that the lacquer is the unintentional source of chromium on the bronzes – and not an old rustproofing treatment. "

The famous Xi'an Terracotta Army is made up of thousands of life-size ceramic figurines depicting warriors stationed in three large pits in the Qin Shihuang Mausoleum (259-210 BC). , the first emperor of a unified China.

These warriors were armed with fully functional bronze weapons. dozens of spears, spears, hooks, swords, crossbows and up to 40,000 arrowheads were found. Although the original organic components of weapons, such as wooden handles, quivers and sheaths, have mostly broken down over the past 2,000 years, the bronze components remain in remarkable condition.

Since the first excavations of the Terracotta Army in the 1970s, the researchers have suggested that the irreproachable state of conservation observed on the bronze weapons was to be the result of development by the manufacturers. Qin weapons of a unique method of preventing the corrosion of metals.

Traces of chrome detected on the surface of the bronze weapons gave the impression that Qin craftsmen set a precedent for chromate conversion coating technology, a technique only patented at the beginning of the 20th century and still used today . The story has been quoted in some books and media.

Now, an international team of researchers has shown that the chromium on bronze surfaces is simply lacquer contamination in adjacent objects and not the result of old technology. The researchers also suggest that the moderately alkaline pH, small particle size and low organic matter in the surrounding soil may have facilitated the conservation of bronze weapons.

Dr. Xiuzhen Li (Institute of Archeology and Terracotta Army Museum of UCL), co-author of the study, said: "Some of the bronze weapons , including swords, spears and halberds, have nearly intact bright surfaces and sharp blades after 2,000 years of burial The hypothesis was that Qin weapons manufacturers could have used some kind of rustproof technology because of the presence of chromium detected on the surface of the weapons, but the preservation of these weapons continued to surprise scientists of forty years.

"The bronze high-tin composition, the quenching technique and the particular nature of the local soil partly explain their remarkable conservation, but it is still possible that the Qin Dynasty developed a mysterious technological process and this deserves a closer investigation. depth. "

By analyzing hundreds of objects, the researchers also discovered that many of the best-preserved bronze weapons did not have surface chromium. To study the reasons for their excellent preservation, they simulated the aging of bronze replicas in a climatic chamber. Bronzes buried in the soil of Xi year have remained almost intact after four months of extreme temperature and humidity, in contrast to the severe corrosion of bronzes buried for comparative purposes in British soil.

"It is striking how much detailed and important information can be retrieved through the evidence of both natural materials and complex artificial recipes found in the mausoleum complex – bronze, clay, wood, lacquer and pigments, to name that these. the history of artisanal production strategies at the dawn of the first Chinese empire, "said the co-author, Professor Andrew Bevan (Institute of Archeology UCL).

Professor Thilo Rehren (Cyprus Institute and Institute of Archeology of UCL) stressed the importance of a long-term collaboration. "We launched this research more than 10 years ago between UCL and the museum.It is only through the persistence, the confident cooperation and the innovative spirit of Chinese and British colleagues that we were able to solve this ten-year-old mystery. "